Retusidae
Housing of Retusa obtusa
The Retusidae are a family of marine snails only in the order of the head shield screws ( Cephalaspidea ). The world's popular small opisthobranch have an outer shell. They feed mainly on small animals such as foraminifera.
Features
The thin-walled, usually not more than 6 mm long shells of Retusidae are pear-shaped, oblong, cylindrical or tapered to the rear, but usually jaded and have a long, extended toward the head of the screw housing estuary. The protoconch is sunken, short and teat- shaped or pointed. The surface of the shell, which has several whorls, is mostly smooth, but may be a slight axially or helically extending sculpturing, or both be present. The worm can retreat completely into its housing, but have most styles no operculum for closing the mouth.
The head shield of Retusidae bears back on the sides a pair of rounded or pointed probe, which often cover the front end of the shell. The jacket does not protrude striking. The snails have neither radula nor jaws, wherein they differ from the very similar Cylichnidae. A gizzard with chewing plates is present, unlike in the earlier part of this family counted genus Rhizorus.
Occurrence and distribution
The approximately 80 species in six genera of Retusidae occur in seas worldwide, from the intertidal and shallow bays to depths of over 5000 m.
The snails of Retusidae feed primarily of foraminifera, but also of very small molluscs ( snails watts ) and of diatoms.
If known, the snails are in the family Retusidae first males and then females ( protandrischer hermaphroditism ) with internal fertilization. At least for some of the species there is a direct development, hatching in the final of the egg capsules worm without prior floating larval stage, so at Retusa obtusa.
System
After Bouchet and Rocroi ( 2005), the family Retusidae one of eight families in the superfamily Philinoidea. This family includes six genera:
- Cylichnina Monterosato, 1884
- Pyrunculus Pilsbry, 1895
- Relichna Rudman, 1971
- Retusa T. Brown, 1827
- Sulcoretusa J.Q. Burch, 1945
- Volvulopsis Schepman, 1913